A study of aerosol optical depth in the central Indian region (17.3-8.6 degrees N) during ISRO-GBP field campaign

We present the ground-based, clear-sky daytime measurements of column aerosol optical depths (AODs) at different wavelengths and the mixing height of atmospheric boundary layers at various locations, conducted during ISRO-GBP aerosol-radiation-trace gases measurements campaign in the central Indian region (17.3-28.6 degrees N and 77.2 degrees-78.2 degrees E) from Delhi to Hyderabad and back, in the month of February 2004. The measurements show AOD variation at 500 nm in the range of 0.2-0.5 and Angstrom exponent in the range of 1.0-1.4 throughout the region. The entire experimental region has been classified into four categories depending upon their locations and surroundings, viz., forest, rural, semi-urban, and rural-dusty. The volume size distribution of the aerosol particles at all these categories showed a bi-modal distribution with fine mode dominating around 0.23 mu m effective radius and the coarse mode dominating around 1 mu m. The forest site showed minimum AOD and alpha values with an equal contribution of aerosol particles in the fine and coarse mode. As the fine-mode particle concentration relatively increased at other regions, the alpha value also went on increasing, along with the increase in AOD. The AOD also showed a latitudinal variation with a minimum occurring at about 23.5 degrees N. During the whole campaign, the average mixing height of planetary boundary layer (PBL) during the daytime was found to be between 650 and 950 m